A person who tries to do somethin, but sucks horrably at it and thinks they are good. Take a skateboarder for instence. If they totally suck and think they are good, you have the right to declare them a poser. They think they are all that and a bag of chips.

"o man i can do a varial kickfip facky."
"Do it!!"
"No, i dont wanna, i dont wanna brag."
"POSER!!!!"